Optical nonlinear crystals are extensively used for frequency conversion of a fundamental laser wavelength to a new wavelength. Examples include conversion of 1064 nm light from neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers to a wavelength of 532 nm using nonlinear crystals such as Potassium Dihydrogen Phosphate (KDP), Barium Borate (BBO), Lithium Triborate (LBO), Bismuth Borate (BiBO), and Potassium Titanyl Phosphate (KTP). This light at 532 nm can be further converted to 355 nm by summing the resultant 532 nm radiation with the remaining laser fundamental at 1064 nm in another crystal to generate 355 nm. The 532 nm can also be converted to 266 nm by doubling in crystals such as BBO, Cesium Dihydrogenarsenate (CDA), Potassium Fluoroboratoberyllate (KBBF) and Cesium Lithium Borate (CLBO). The 266 nm can be converted by summing with the fundamental at 1064 nm to get to wavelengths as short as 213 nm.
It is theoretically possible to attain conversion efficiencies of the fundamental laser wavelength to the desired wavelength range by as high as 100% for flat top spatial/temporal laser pulses. In practice, conversion efficiencies as high as 80-90% for second harmonic generation (SHG) and 30-40% for third harmonic generation (THG) to ultraviolet (UV) are attained using spatial-temporal shaped pulses and/or effective multi-pass operation of the nonlinear crystals. In practice, solid state lasers have been scaled to increasingly higher powers to attain significant brightness. For example, it is possible to produce continuous lasers with diffraction limited output at 1 KW, which corresponds to a focusable average power of more than 1011 W/cm2. Despite the high power scaling capabilities of the fundamental laser source, it has not been possible to scale the nonlinear frequency conversion to take full advantage of the higher power inputs.
The surface damage is especially relevant if the laser and nonlinear crystal are arranged to generate ultraviolet (UV) laser radiation. Most available nonlinear crystals have a significantly lower damage threshold when exposed to UV laser radiation compared to lower frequency laser radiation. As a result, high intensity of UV radiation on the exit face of a nonlinear crystal causes rapid degradation of that component, limiting the reliability of the laser system. Thus, there is a need in the art for methods and systems for reducing surface damage in nonlinear crystals used for efficient, high power frequency conversion of laser light.
An object of the present invention is to provide a method of reducing surface damage in a nonlinear crystal.